Frequency Resolved Optical Gating (FROG) apparatus is used to measure the amplitude and phase, versus frequency of an incoming beam of pulsed laser light. Such apparatus is popular amongst experimenters partly because of the simplicity of the setup and partly due to the error checking robustness of its algorithm, which is not present in the alternative method known as SPIDER (Spectral Phase Interferometry for Direct Electric-field Reconstruction). The electric field can be uniquely described by, E=A eiφ, where A is the amplitude and φ is the phase, and they both depend on the frequency (or wavelength).
The phrase “frequency resolved optical gating” results from a theory that a short gate pulse can be used to obtain a sample from a longer pulse by nonlinear mixing (gating) in a nonlinear crystal material. Since a gate pulse shorter than the pulse to be measured is not usually available, FROG uses the pulse itself for gating.
There are different versions of FROG that rely upon different nonlinear gating mechanisms, that generate different kinds of FROG traces (requiring different phase retrieval algorithms), and that have different strengths and weaknesses. Polarization gated FROG (PG FROG) is the simplest known version, and uses a polarizer with a high extinction ratio to produce a signal that is analyzed by a spectrometer.
The self-difference FROG (SD FROG) version utilizes two overlapping beams in an X(3) medium to generate a nonlinear refractive index grating. The grating diffracts both overlapping beams into two new beams, one of which is used for detection. A disadvantage to this FROG version is that relatively large pulse energies are required for its operation.
The Transient-Grating FROG (TG FROG) version is similar to the SD FROG, but uses a third pulse with a variable delay as a probe that is diffracted at the grating generated by the other two beams to provide a much higher detection sensitivity than SD FROG.
The Second-Harmonic FROG (SHG FROG) version is the most popular FROG version and is based upon an X(2) nonlinear crystal that allows the apparatus to achieve a much higher sensitivity than is possible with X(3) crystals. Phase matching issues require careful treatment to avoid distortion for short pulses.
The Interferometric FROG (IFROG) version uses a collinear geometry to avoid a loss of temporal resolution via geometric effects (finite beam angles) in the characterization of few-cycle pulses.
One existing method for phase matching very weak pulses of laser light is to dither a thick crystal in front of the beam. This method, however, introduces several problems. Since the incident angle is exposed to varying thicknesses of the crystal, dependent on the angle of incidence, the efficiency and the bandwidth of the SHG varies. The crystal also requires a supporting geometry with a mechanism to introduce acceleration and deceleration for a continuous acquisition. Control of such continuous change in motion of the crystal is difficult to achieve and requires that the acceleration and deceleration be incorporated into measurements when a velocity control mode is used. Also, the changing angle between the incident light and the crystal surface results in varying reflectivity off the surface of the crystal. Also, as the crystal rotates, the beam's spot size inside the crystal varies resulting in non-uniform SHG efficiencies.